Add missing Q_OBJECT macros in header files

Fixes 20 warnings similar to the following ones when generating a translation file with e.g. "make de.ts":
/home/daniel/Lmms/src/tracks/AutomationTrack.cpp:40: Class 'AutomationTrack' lacks Q_OBJECT macro
/home/daniel/Lmms/src/core/FxMixer.cpp:41: Class 'FxRoute' lacks Q_OBJECT macro
/home/daniel/Lmms/src/core/FxMixer.cpp:554: Class 'FxMixer' lacks Q_OBJECT macro
/home/daniel/Lmms/src/gui/widgets/MeterDialog.cpp:52: Class 'MeterDialog' lacks Q_OBJECT macro
...

src/gui/widgets/InstrumentMidiIOView.cpp

@@ -201,4 +201,4 @@ void InstrumentMidiIOView::modelChanged()
 	}
 }
 
-
+#include "moc_InstrumentMidiIOView.cxx"

src/gui/widgets/MeterDialog.cpp

@@ -113,4 +113,4 @@ void MeterDialog::modelChanged()
 	m_denominator->setModel( &mm->denominatorModel() );
 }
 
-
+#include "moc_MeterDialog.cxx"

src/tracks/AutomationTrack.cpp

@@ -192,4 +192,4 @@ void AutomationTrackView::dropEvent( QDropEvent * _de )
 }
 
 
-
+#include "moc_AutomationTrack.cxx"